1. Field of the Invention
The present invention concerns an apparatus for the producing compacts having layers of different composition and, in particular, to an apparatus where each of the different metal powder layers of a compact is charged in a pressing tool and pressed before the following metal powder layer is charged.
2. Description of the Prior Art
A method for making compacts is disclosed in German Pat. Application No. 1,458,477. In this method multi-layer compacts, which are used for making heavy duty electric contacts, are pressed layer by layer, then sintered and impregnated with a metal of high electric conductivity. To give the layered sinter skeleton a different degree of packing, the individual superposed layers of the compact are compressed with different pressing pressures.
The aforesaid production of the multi-layer compacts is usually carried out by employing a pressing tool which is fixed on a press table and which includes a punch, die and die-plate. Between the individual strokes of the pressing tool, charging devices are moved over the die-plate, charging the respective metal powder layers. With this technique it is unavoidable that scattered powder lying on the press table from previous chargings will get into the subsequent metal powder layers to be charged. This results in an undesired mixing of the different metal powder layers. Additionally, since succeeding strokes of press follow rapidly, the press cannot be readily adjusted to provide different constant pressing pressures for different powder layers.
In the aforesaid prior art method, moreover, mechanical presses must necessarily be employed, since hydraulic units can only be adjusted to certain pressing pressures which lie between the minimum pressure and the maximum pressure of the respective hydraulic unit. In using such mechanical presses, the stroke is adjusted so that the individual metal powder layers are compacted to a certain height. When pressing to constant height, however, a constant pressing pressure is not maintained, and, due to the tolerances in the charging of the metal powder layers, this results in density variations in the individual layers of the compacts. With the occurrence of these density fluctuations, the conditions for the subsequent sintering and impregnating of the compacts can no longer be fixed optimally. Moreover, the density fluctuations have an effect on the degree of packing of the impregnating metal, so that the electric contacts produced with the compacts show quality fluctuations with regard to their resistance to burning and their electric conductivity.
It is also known that tablets having two or more layers can be made by moving each of a plurality of pressing tools past work stations located on a circular path and at which charging of the different composition occurs. Between these work stations the punches of the pressing tools are pushed down over a punch cam until the previously charged respective layers have been compressed to a certain height. Since constant density of the layers of the different tablets is not important, the aforesaid known technique is not applicable to the making of the above-described compacts for heavy duty electric contacts. Moreover, since in the tablet making procedure, the pressing tools are moved continuously, one cannot realize constant pressing pressures through the technique of pressing to a constant height.
The primary object of the present invention is to provide an apparatus for producing compacts having layers of different composition in which the individual layers are neatly separated and not intermixed and in which no density fluctuations occur in the individual layers of different compacts.